In some structures, the energy of an impact by an object on a first surface of a structure may manifest in cracks that open onto a second opposing surface of the structure. This phenomenon is often referred to as back face damage, as it is the “back face” relative to the impact site that exhibits such damage.
Composite materials, which are often referred to simply as “composites”, are now commonly used in high technology applications. One common example of a composite is fibre reinforced plastics, which has fibres embedded in a cured resin. Composites have the advantages of high strength and stiffness for their unit mass.
One disadvantage of composites is their poor resistance to impact damage. As composites are commonly used in a sheet or sheet-like form, impact damage typically occurs in the through thickness direction. Composites exhibit little or no plastic deformation, such that impact energy is largely absorbed as either elastic deformation or structural damage. Such structural damage can occur internally within the composite material. Alternatively, the damage can manifest itself on the opposing surface to the surface receiving the actual impact. In such cases, the surface receiving the impact may show little or no evidence of either impact or damage.
Such damage is often referred to as Barely Visible Impact Damage (BVID), as the damage is often difficult or impossible to visually observe by the naked eye. Often the only evidence of impact that can be observed visually may be fine cracks that are created on the opposing surface to the actual impact.